Method for finding and tracking single-mode operation point of external cavity diode lasers

ABSTRACT

An apparatus comprising: a processor for determining if a laser is operating in a single-mode state and for determining the degree to which one of one or more tunable parameters for the laser must be adjusted so that laser operates in a single-mode state if not operating in a single-mode state, wherein the one or more tunable parameters include the following parameters: the laser current and the wavelength of the output light. The apparatus may include a laser and/or a holographic storage medium. Also provided is a method for determining if a laser is operating in a single-mode state and for determining the degree to which one of one or more tunable parameters for the laser must be adjusted so that laser operates in a single-mode state if not operating in a single-mode state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to and claims the benefit of thefollowing co-pending U.S. Provisional Patent Application No. 61/098,445filed Sep. 19, 2008. The entire disclosure and contents of the foregoingProvisional Application is hereby incorporated by reference. Thisapplication also makes reference to the following U.S. patentapplications: U.S. patent application Ser. No. 12/508,784 entitled“HOLOGRAPHIC STORAGE MEDIUM AND METHOD FOR GATED DIFFUSION OFPHOTOACTIVE MONOMER,” filed Jul. 24, 2009. U.S. patent application Ser.No. 12/506,284 entitled “METHOD TO MODIFY AND APPLY EDGE SEAL MATERIALSIN LAMINATED MEDIA,” filed Jul. 21, 2009. U.S. patent application Ser.No. 12/457,498 entitled “SYSTEM AND DEVICES FOR IMPROVING EXTERNALCAVITY DIODE LASERS USING WAVELENGTH AND MODE SENSORS AND COMPACTOPTICAL PATHS,” filed Jun. 12, 2009. U.S. Provisional Patent ApplicationNo. 60/980,604 entitled “LAYOUT METHOD FOR MULTIPLEXED HOLOGRAMS” filedOct. 17, 2007. U.S. Provisional Patent Application No. 61/083,254,entitled “METHOD ALLOWING LOCALIZED GATING OF DIFFUSION PROPERTIES,”filed Jul. 24, 2008; U.S. Provisional Patent Application No. 61/082,328,entitled “METHOD TO MODIFY AND APPLY EDGE SEAL MATERIALS TO LAMINATEDMEDIA SO THAT THE RESULTING SEAL HAS MINIMAL EFFECT ON THE SHAPE OF THEMEDIA AFTER EXPOSURE TO ELEVATED TEMPERATURES,” filed Jul. 21, 2008;U.S. Provisional Patent Application No. 61/060,890, entitled “SYSTEM ANDDEVICES FOR IMPROVING EXTERNAL CAVITY DIODE LASERS USING WAVELENGTH ANDMODE SENSORS AND COMPACT OPTICAL PATHS,” filed Jun. 12, 2008; U.S.Provisional Patent Application No. 61/054,613, entitled “METHOD FORCOMPENSATING FOR THERMAL EFFECTS OF A PHOTOPOLYMER BY USING ADAPTIVEENERGY CONTROL,” filed May 20, 2008; U.S. Provisional Patent ApplicationNo. 61/028,628, entitled “SERVO FOR HOLOGRAPHIC DATA STORAGE,” filedFeb. 14, 2008. U.S. Provisional Patent Application No. 60/980,604entitled “LAYOUT METHOD FOR MULTIPLEXED HOLOGRAMS” filed Oct. 17, 2007;U.S. patent application Ser. No. 12/210,476, entitled “LAYOUT METHOD FORMULTIPLEXED HOLOGRAMS” filed Sep. 15, 2008; U.S. Provisional PatentApplication No. 60/855,754, entitled “EMULATION OF DISSIMILAR REMOVABLEMEDIUM STORAGE DEVICE TYPES ASSISTED BY INFORMATION EMBEDDED IN THELOGICAL FORMAT,” filed Sep. 1, 2006; U.S. patent application Ser. No.11/849,658, entitled “EMULATION OF DISSIMILAR REMOVABLE MEDIUM STORAGEDEVICE TYPES ASSISTED BY INFORMATION EMBEDDED IN THE LOGICAL FORMAT,”filed Sep. 4, 2007; U.S. Provisional Patent Application No. 60/831,692,entitled “EXTERNAL CAVITY DIODE LASER COLLIMATION GROUP ADJUSTMENT”filed Jul. 19, 2006; U.S. patent application Ser. No. 11/826,517,entitled “COLLIMATION LENS GROUP ADJUSTMENT FOR LASER SYSTEM” filed Jul.16, 2007; U.S. Provisional Patent Application No. 60/802,530, entitled“HIGH-SPEED ELECTROMECHANICAL SHUTTER” filed May 25, 2006; U.S. patentapplication Ser. No. 11/752,804, entitled “HIGH-SPEED ELECTROMECHANICALSHUTTER” filed May 25, 2007; U.S. Provisional Patent Application No.60/793,322, entitled “METHOD FOR DESIGNING INDEX CONTRASTING MONOMERS”filed Apr. 20, 2006; U.S. patent application Ser. No. 11/738,394,entitled “INDEX CONTRASTING-PHOTOACTIVE POLYMERIZABLE MATERIALS, ANDARTICLES AND METHODS USING SAME” filed Apr. 20, 2007; U.S. ProvisionalPatent Application No. 60/780,354, entitled “EXTERNAL CAVITY LASER”filed Mar. 9, 2006; U.S. patent application Ser. No. 11/716,002,entitled “EXTERNAL CAVITY LASER” filed Mar. 9, 2007; U.S. ProvisionalPatent Application No. 60/779,444, entitled “METHOD FOR DETERMININGMEDIA ORIENTATION AND REQUIRED TEMPERATURE COMPENSATION IN PAGE-BASEDHOLOGRAPHIC DATA STORAGE SYSTEMS USING DATA PAGE BRAGG DETUNINGMEASUREMENTS” filed Mar. 7, 2006; U.S. patent application Ser. No.11/714,125, entitled “METHOD FOR DETERMINING MEDIA ORIENTATION ANDREQUIRED TEMPERATURE COMPENSATION IN PAGE-BASED HOLOGRAPHIC DATA STORAGESYSTEMS USING DATA PAGE BRAGG DETUNING MEASUREMENTS” filed Mar. 6, 2007;U.S. Provisional Patent Application No. 60/778,935, entitled “MINIATUREFLEXURE BASED SCANNERS FOR ANGLE MULTIPLEXING” filed Mar. 6, 2006; U.S.Provisional Patent Application No. 60/780,848, entitled “MINIATUREFLEXURE BASED SCANNERS FOR ANGLE MULTIPLEXING” filed Mar. 10, 2006; U.S.Provisional Patent Application No. 60/756,556, entitled “EXTERNAL CAVITYLASER WITH A TUNABLE HOLOGRAPHIC ELEMENT” filed Jan. 6, 2006; U.S.patent application Ser. No. 11/649,801, entitled “AN EXTERNAL CAVITYLASER WITH A TUNABLE HOLOGRAPHIC ELEMENT” filed Jan. 5, 2007; U.S.Provisional Patent Application No. 60/738,597, entitled “METHOD FORHOLOGRAPHIC DATA RETRIEVAL BY QUADRATURE HOMODYNE DETECTION” filed Nov.22, 2005; U.S. patent application Ser. No. 11/562,533, entitled “METHODFOR HOLOGRAPHIC DATA RETRIEVAL BY QUADRATURE HOMODYNE DETECTION” filedNov. 22, 2006; U.S. patent application Ser. No. 11/402,837, entitled“ARTICLE COMPRISING HOLOGRAPHIC MEDIUM BETWEEN SUBSTRATES HAVINGENVIRONMENTAL BARRIER SEAL AND PROCESS FOR PREPARING SAME” filed Dec. 2,2005; U.S. patent application Ser. No. 11/291,845, entitled “ARTICLECOMPRISING HOLOGRAPHIC MEDIUM BETWEEN SUBSTRATES HAVING ENVIRONMENTALBARRIER SEAL AND PROCESS FOR PREPARING SAME” filed Dec. 2, 2005; U.S.Provisional Patent Application No. 60/728,768, entitled “METHOD ANDSYSTEM FOR INCREASING HOLOGRAPHIC DATA STORAGE CAPACITY USINGIRRADIANCE-TAILORING ELEMENT” filed Oct. 21, 2005; U.S. patentapplication Ser. No. 11/319,425, entitled “METHOD AND SYSTEM FORINCREASING HOLOGRAPHIC DATA STORAGE CAPACITY USING IRRADIANCE-TAILORINGELEMENT” filed Dec. 27, 2005; U.S. Provisional Application No.60/684,531, entitled “METHODS FOR MAKING A HOLOGRAPHIC STORAGE DRIVESMALLER, CHEAPER, MORE ROBUST AND WITH IMPROVED PERFORMANCE” filed May26, 2005; U.S. patent application Ser. No. 11/440,368, entitled“REPLACEMENT AND ALIGNMENT OF LASER” filed May 25, 2006. U.S.application Ser. No. 11/440,369, entitled “HOLOGRAPHIC DRIVE HEADALIGNMENTS” filed May 25, 2006; U.S. patent application Ser. No.11/440,365, entitled “LASER MODE STABILIZATION USING AN ETALON” filedMay 25, 2006; U.S. patent application Ser. No. 11/440,366, entitled“ERASING HOLOGRAPHIC MEDIA” filed May 25, 2006; U.S. patent applicationSer. No. 11/440,367, entitled “POST-CURING OF HOLOGRAPHIC MEDIA” filedMay 25, 2006; U.S. patent application Ser. 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No. 11/440,446, entitled “METHODS ANDSYSTEMS FOR LASER MODE STABILIZATION” filed May 25, 2006; U.S. patentapplication Ser. No. 11/440,370, entitled “ILLUMINATIVE TREATMENT OFHOLOGRAPHIC MEDIA” filed May 25, 2006; U.S. patent application Ser. No.11/447,033, entitled “LOADING AND UNLOADING MECHANISM FOR DATA STORAGECARTRIDGE AND DATA DRIVE” filed Jun. 6, 2006; U.S. patent applicationSer. No. 11/283,864, entitled “DATA STORAGE CARTRIDGE LOADING ANDUNLOADING MECHANISM, DRIVE DOOR MECHANISM AND DATA DRIVE” filed Nov. 22,2006; U.S. patent application Ser. No. 11/237,883, entitled “HOLOGRAPHICRECORDING MEDIUM AND SUBSTRATE WITH CTE COMPENSATING INTERFACETHEREBETWEEN” filed Sep. 29, 2005; U.S. patent application Ser. No.11/261,840, entitled “SHORT STACK RECORDING IN HOLOGRAPHIC MEMORYSYSTEMS” filed Dec. 2, 2005; U.S. patent application Ser. No.11/067,010, entitled “HIGH FIDELITY HOLOGRAM DEVELOPMENT VIA CONTROLLEDPOLYMERIZATION” filed Feb. 28, 2005; U.S. Provisional Application No.60/576,381, entitled “METHOD FOR ORGANIZING AND PROTECTING DATA STOREDON HOLOGRAPHIC MEDIA BY USING ERROR CONTROL AND CORRECTION TECHNIQUESAND NEW DATA ORGANIZATION STRUCTURES” filed Jun. 3, 2004; U.S. patentapplication Ser. No. 11/139,806, entitled “DATA PROTECTION SYSTEM” filedMay 31, 2005. U.S. application Ser. No. 11/140,151, entitled“MULTI-LEVEL FORMAT FOR INFORMATION STORAGE” filed May 31, 2005; U.S.patent application Ser. No. 10/866,823, entitled “THERMOPLASTICHOLOGRAPHIC MEDIA” filed Jun. 15, 2004. The entire disclosure andcontents of the foregoing U.S. patent applications are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention broadly relates generally to external cavity laserdiode (ECLD) systems and methods.

BACKGROUND

Developers of information storage devices continue to seek increasedstorage capacity. As part of this development, holographic memorysystems have been suggested as alternatives to conventional memorydevices. Holographic memory systems may be designed to record data onebit of information (i.e., bit-wise data storage). See McLeod et al.“Micro-Holographic Multi-Layer Optical Disk Data Storage,” InternationalSymposium on Optical Memory and Optical Data Storage (July 2005).Holographic memory systems may also be designed to record an array ofdata that may be a 1-dimensional linear array (i.e., a 1×N array, whereN is the number linear data bits), or a 2-dimension array commonlyreferred to as a “page-wise” memory systems. Page-wise memory systemsmay involve the storage and readout of an entire two-dimensionalrepresentation, e.g., a page of data. Typically, recording light passesthrough a two-dimensional array of dark and transparent areasrepresenting data, and the system stores, in three dimensions, the pagesof data holographically as patterns of varying refractive indeximprinted into a storage medium. See Psaltis et al., “HolographicMemories,” Scientific American, November 1995, where holographic systemsare discussed generally, including page-wise memory systems.

In a holographic data storage system, information is recorded by makingchanges to the physical (e.g., optical) and chemical characteristics ofthe holographic storage medium. These changes in the holographic storagemedium take place in response to the local intensity of the recordinglight. That intensity is modulated by the interference between adata-bearing beam (the data beam) and a non-data-bearing beam (thereference beam). The pattern created by the interference of the databeam and the reference beam forms a hologram which may then be recordedor written in the holographic storage medium. If the data-bearing beamis encoded by passing the data beam through, for example, a spatiallight modulator (SLM), the hologram(s) may be recorded or written in theholographic storage medium as holographic data.

External cavity laser diodes (ECLDs) are useful light sources forapplications in spectroscopy, telecommunications and holography.Holographic data storage also illustrates an application with threerequirements that an ECLD meets: wide wavelength tuning range, operationin a single-longitudinal mode, and output powers in the tens ofmilliwatts. In some holographic data storage approaches, the operatingwavelength range may be in the range of from about 402 to about 408 nm.Since holograms are created by interference, single-longitudinal modeoperation may be necessary to form holograms having a highsignal-to-noise ratio. Finally, the created holograms, which are storedin a holographic storage medium, depend upon the number of photonsdelivered to the storage medium.

SUMMARY

According to a first broad aspect of the present invention, there isprovided an apparatus comprising: a laser having a laser current and anoutput light having a wavelength; and a processor for determining if thelaser is operating in a single-mode state and for determining the degreeto which one of one or more tunable parameters for the laser must beadjusted so that laser operates in a single-mode state if not operatingin a single-mode state, wherein the one or more tunable parametersinclude the following parameters: the laser current and the wavelengthof the output light.

According to a second broad aspect of the present invention, there isprovided a method comprising the following steps: (a) determining if alaser is operating in a single-mode state, the laser having a lasercurrent and an output light having a wavelength; and (b) if the laser isdetermined to not be operating in a single-mode state in step (a),determining the degree to which one of one or more tunable parametersfor the laser must be adjusted so that the laser operates in asingle-mode state if not operating in a single-mode state, wherein theone or more tunable parameters include the following parameters: thelaser current and the wavelength of the output light.

According to a third broad aspect of the present invention, there isprovided an apparatus comprising: a holographic storage medium forrecording holograms using a tunable laser having a laser current and anoutput light having a wavelength; and a processor for determining if thelaser is operating in a single-mode state and for determining the degreeto which one of one or more tunable parameters for the laser must beadjusted so that laser operates in a single-mode state if not operatingin a single-mode state, wherein the one or more tunable parametersinclude the following parameters: the laser current and the wavelengthof the output light.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a histogram of a range of contrast ratios made from multiplecurrent sweeps while tuning the wavelength of the external cavity laserdiode;

FIG. 2 is a plot of the contrast ratio versus the external cavity laserdiode (ECLD) current, wherein the ECLD current is tuned within the rangeof from about 84.5 mA to about 92.8 mA;

FIG. 3 is a plot of the contrast ratio versus the ECLD current tunedwithin the current range of from about 84.5 mA to about 92.8 mA for upand down ECLD current sweeps, and indicating the chosen operatingcurrent for the ECLD of the widest overlapping single-mode plateau ofthe up and down current sweeps; and

FIG. 4 is a flowchart illustrating an Automatic Mode Control (AMC)process according to one embodiment of the present invention.

DETAILED DESCRIPTION

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

DEFINITIONS

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, directional terms such as“top,” “bottom,” “above,” “below,” “left,” “right,” “horizontal”“vertical,” “up,” “down,” etc., are merely used for convenience indescribing the various embodiments of the present invention. Theembodiments of the present invention may be oriented in various ways.For example, the devices, diagrams, graphs, images, etc., shown in FIGS.1 through 3 may be flipped over, rotated by 90° in any direction, orreversed, etc.

For the purposes of the present invention, a value or property is“based” on a particular value, property, the satisfaction of acondition, or other factor, if that value is derived by performing amathematical calculation or logical decision using that value, property,condition, or other factor.

For the purposes of the present invention, the term “angle of incidence”refers to the angle between a light ray incident on a surface and theline perpendicular to that surface (the normal) at the point ofincidence.

For the purposes of the present invention, the term “Automatic ModeControl (AMC) process” refers to a process which may be used to keep alaser, such as an ECLD, tuned to operate as a single-mode laser. In oneembodiment of the present invention, the AMC process starts to adjustcurrent and/or wavelength when the contrast ratio of a laser is belowthe set point threshold.

For the purposes of the present invention, the term “AMC current range”refers to a parameter that refers to the amount that the current isadjusted for the laser before the wavelength of the laser is adjusted bythe AMC process in one embodiment of the present invention.

For the purposes of the present invention, the term “current step”refers to a constant amount by which the AMC process adjusts lasercurrent in embodiments of the present invention. In one embodiment, thecurrent step is 50 microamps (mA).

For the purpose of the present invention, the terms “contrast ratio” or“fringe visibility” (also known as “interference visibility” or“interferometric visibility”) refer interchangeably to the quantifiedcontrast of an interference (fringe pattern) in a system which haswave-like properties. Generally, when two or more waves are combined andas the phase between them is changed (e.g., in an interferometer), thepower or intensity of the resulting wave oscillates, thus forming aninterference pattern. The ratio of the size or amplitude of theseoscillations to the sum of the powers of the individual waves is definedas the visibility or contrast ratio. In one embodiment of the presentthe contrast ratio is obtained by comparing a scaled contrast value to ascaled maximum contrast value. In an embodiment, the contrast value mayvary from 0 to 1023. However, the measured contrast value is adjusted tofit as scale of 0 to 736 for generating scaled contrast value bydividing the contrast value on the 0 to 1023 scale by 1.39. The scalingof the contrast value is performed to allow for using a FourierTransform method to calculate the contrast ratio of a fringe pattern,which is more accurate than directly measuring the maximum and minimumof a fringe pattern. The Fourier transform method yields contrast ratiovalues that are precisely lower than the correct contrast ratio by ascale factor, as described using techniques such as those described incommonly assigned U.S. patent application Ser. No. 12/457,498, entitled“SYSTEM AND DEVICES FOR IMPROVING EXTERNAL CAVITY DIODE LASERS USINGWAVELENGTH AND MODE SENSORS AND COMPACT OPTICAL PATHS” (Ensher et al.),filed Jun. 12, 2009, the entire contents and disclosure of which ishereby incorporated by reference.

For the purposes of the present invention, the term “contrast ratioplateau” refers to contiguous series of currents for a laser where thecontrast ratio is relatively constant as the laser current changes. Inone embodiment of the present invention, the contrast ratio isconsidered “constant” if the contrast ratio differs by about 13 units(on a scale of 0 to 736 units) or less. Examples of contrast ratioplateaus are shown in FIGS. 2 and 3.

For the purposes of the present invention, the term “current updatedelay” refers to the time between adjusting the laser current and whenthe status of the laser is checked during the AMC process according toone embodiment of the present invention.

For the purposes of the present invention, the term “diffractiongrating” refers to an optical component whose optical properties may beperiodically modulated and which results in the incoming light exitingthe grating with an angle which is dependent upon the wavelength of theincident light. Diffraction gratings have a regular or repeating patternwhich can split (diffract) light into a plurality of beams travelling indifferent directions. Diffraction gratings may be reflective ortransmissive.

For the purposes of the present invention, the term “external lasercavity” refers to a laser cavity which is external to a component of anECLD which is the source of photons and optical gain. Exemplary externallaser cavities comprise the portion of an ECLD between a laser diode anda diffraction grating (including any collimating lens positioned betweenthe laser diode and the diffraction grating), etc. External lasercavities often provide control over the longitudinal and/or transversemode structure of the laser diode of the ECLD.

For the purposes of the present invention, the term “fringe pattern”refers to the pattern of interference fringes formed by the interaction,intersection, and/or interference, etc., of two or more light beams.Fringe patterns are illustrated, for example, in FIGS. 4,5 and 13, aswell as the corresponding description, in commonly-assigned U.S. Pat.No. 7,397,571 (Krneta et al.), issued Jul. 8, 2008, the entiredisclosure and contents of which is hereby incorporated by reference

For the purposes of the present invention, the term “full-width halfmaximum” (FWHM) refers to an expression of the extent of a function,given by the difference between the two extreme values of theindependent variable at which the dependent variable is equal to half ofthe maximum value of the dependent variable.

For the purposes of the present invention, the term “initial wavelength”refers to the initial wavelength of a laser. In one embodiment of thepresent invention, the AMC process has an initial wavelength which isset and which is initially held constant as the laser current isadjusted. If the laser is at the initial wavelength for the laser andthe amount of current adjustment necessary to achieve single-modeoperation equals the maximum current range, the initial wavelength isadjusted by a wavelength step. In one embodiment, the initial wavelengthmay be from 402 to 408 nm. The initial wavelength may be affected by thetemperature of the holographic medium in which the laser recordsholograms.

For the purposes of the present invention, the term “laser current”refers to the current applied to the laser diode. In the AMC process ofthe present invention, the contrast ratio for the output light of alaser may be adjusted by adjusting the laser diode current.

For the purposes of the present invention, the term “laser wavelengthprecision value” refers to the tolerance that is allowed between therequested laser wavelength and the final laser wavelength. In oneembodiment of the present invention, a set wavelength request in the AMCprocess invokes a process that adjusts the grating to converge to therequested wavelength value until the actual value is within the laserwavelength precision of the requested wavelength.

For the purposes of the present invention, the term “maximum currentrange” refers to an amount that the laser current may be changed beforeit is necessary to bump the wavelength of the output light of a laserwhen employing the AMC process of one embodiment of the presentinvention. The “maximum current range” may be determined by theresulting change in the output power of the laser. For instance, a 1 mAchange in the laser diode current may cause a 0.5 mW change in theoutput power of the laser. Over a certain current range, the change inoutput power of the laser may not change the signal to noise strength ofthe holograms produced (the range is precisely dependent on details ofthe holographic drive such as amount of light actually delivered to aholographic media, the amount of scattered light produced, etc.). In oneembodiment, the maximum current range is 3000 microamps (mA).

For the purposes of the present invention, the term “maximum wavelengthrange” refers to the total change in wavelength that the AMC process isallowed to perform before indicating that the AMC process has failed orthat the algorithm must return to the initial wavelength requested andtry again. The maximum wavelength range for a laser may be determined bythe wavelength-dependent response of the holographic media, which mayexhibit a peak response over hundreds of picometers.

For the purposes of the present invention, the term “mode hop” refers toan integral change in the of longitudinal modes supported by a lasercavity. A mode hop may occur as the ECLD wavelength or laser diodecurrent are tuned due to a change in the cavity length or change in thewavelength of light that is selected by the grating to be supported bythe cavity.

For the purposes of the present invention, the term “mode number” refersto the number of half wavelengths of a particular wavelength of lightthat fits within a laser cavity.

For the purposes of the present invention, the term “non-output beam”refers to a beam produced by, for example, an ECLD which do not provideoutput from the laser cavity. Non-output beams may include, for example,specularly reflected beams (R0), reflected diffraction order beams(R−1), and/or transmitted diffraction order beams (T1).

For the purposes of the present invention, the term “currentoptimization procedure” refers to a portion of the AMC process in whichone or more contrast ratio plateaus above the control point thresholdare located and the laser current is set in the middle of the plateauthereby defining an “optimized (laser) current” for the laser. In oneembodiment of the present invention, the optimizing current proceduresets the operating current for the laser at the approximate midpoint ofthe largest plateau above the control point threshold if a singlecurrent sweep is performed on the laser or at the midpoint of thelargest overlapping plateau if multiple current sweeps are performed. Inone embodiment of the present invention, the AMC process sweeps over arange of 3000 microamps (3 mA) to determine an optimized current.

For the purposes of the present invention, the term “overlappingsingle-mode (SM) plateau” refers the a current range where two or moreSM plateaus overlap.

For the purposes of the present invention, the term “position sensitivedetector (PSD)” refers to a device which detects and enables positionmeasurement to be made, determined, and/or calculated, etc. The PSD maybe one-dimensional (linear), two-dimensional, or three-dimensional. PSDsmay include a photodiode array, e.g., a bicell or quad cell photodiode;a diffraction grating sensor; CMOS camera; and a CCD, e.g., a CCD lineararray, etc.

For the purposes of the present invention, the term “reflecteddiffraction order beam (R1)” refers to a beam produced by thediffraction grating of an ECLD which is often used to provide feedbackto the laser diode.

For the purposes of the present invention, the term “reflectivediffraction grating” refers to a diffraction grating in which all or atleast most of the light which reaches the grating is reflected.Reflective diffraction gratings comprise a reflective surface, coating,or substrate, etc., which permits the non-diffracted light to bereflected from the substrate.

For the purposes of the present invention, the term “initial power”refers to the initial output power level to which an external cavitylaser in an AMC process, according to one embodiment of the presentinvention, is set. In one embodiment, the initialization part of the AMCprocess attempts to set a starting requested power as the initial powerfor a laser. If the initialization part of the AMC process determinesthat the laser cannot be operated in single-mode at the startingrequested power, even with adjustments to the laser current, the AMCprocess selects a new target power and tries to set a new startingrequested power that as the initial power. This process is repeateduntil the initialization part of the AMC process determines that thelaser can be operated, possibly with laser current adjustments, insingle-mode at the starting requested power and sets the startingrequested power as the initial power for the laser.

For the purposes of the present invention, the term “sensor array”refers to a set of several sensors which an information gathering deviceuses to gather data which may not be gathered from a single source.

For the purposes of the present invention, the term “shearinginterferometer” refers to a testing device which comprises a plate madeof, for example, a high quality optical glass (e.g., BK-7) withextremely flat optical surfaces and usually having a slight anglebetween them (e.g., is wedge-shaped). When a plane wave is incident tothe glass plate at an angle of 45 degrees (which gives maximumsensitivity) it is reflected twice, with the two reflections beinglaterally separated due to the finite thickness of the plate and by thewedge shape of the plate. This “separation” is referred to as the“shear” which gives the interferometer its name. Shearinginterferometers may be used to observe interference and to use thisphenomenon to test the collimation of light beams, especially from lasersources (e.g., laser diodes of ECLDs) which have a coherence lengthwhich may be a lot longer than the thickness of the shear plate so thatthe basic condition for interference is fulfilled.

For the purposes of the present invention, the term “single-mode (SM)plateau” refers to a contrast ratio plateau in a region where an ECLDoperates as a single-mode laser. Examples of SM plateaus are shown inFIGS. 2 and 3.

For the purposes of the present invention, the term “specularlyreflected beam (R0)” refers to a beam produced by the diffractiongrating of an ECLD which provides a minor-like reflection of the lightoriginally impacting upon the grating.

For the purposes of the present invention, the term “transmissivediffraction grating” refers to a diffraction grating which permits aportion of the light to pass through the grating. Transmissivediffraction gratings comprise a transparent material, element,component, structure, and/or substrate, etc., which permits thenon-diffracted light to be transmitted (pass) through the substrate.Exemplary transmissive diffraction gratings may comprise devices capableof diffracting a portion of light at a particular wavelength whichpasses through the device back along the same path upon which theincoming light traveled.

For the purposes of the present invention, the term “transmitted beam(T0)” refers to a beam produced by the diffraction grating of an ECLDwhich provides output (an output beam) from the laser cavity.

For the purposes of the present invention, the term “transmitteddiffraction beam (T1)” refers to a beam produced by the diffractiongrating of an ECLD which passes through (is transmitted by) the grating.

For the purposes of the present invention, the term “tune” refers toadjusting a device to a desired state. For example, in exemplaryembodiments, a diffraction grating may be tuned by adjusting theparticular wavelength reflected (or transmitted) by the diffractiongrating to a desired wavelength. In other embodiments, the device may betuned adjusting and controlling the degree of coherence of the lasermode.

For the purposes of the present invention, the terms “laser cavity,”“optical cavity,” “optical resonator,” or “laser resonator” (hereaftercollectively referred to as “laser cavity”) refers to a space betweentwo reflective devices, elements, etc., of an ECLD. Exemplary lasercavities may comprise the space between, for example, the space betweenreflective coatings on a facet of a laser diode, the space between alaser diode and a diffraction grating, etc.

For the purposes of the present invention, the term “coherent lightbeam” refers to a beam of light including waves with a particular (e.g.,constant) phase relationship, such as, for example, a laser beam.

For the purposes of the present invention, the term “light source”refers to a source of electromagnetic radiation having a singlewavelength or multiple wavelengths. The light source may be from alaser, a laser diode, and/or a light emitting diode (LED), etc.

For the purposes of the present invention, the term “bump” refers toadjusting a wavelength of a laser by the wavelength step for the laser.

For the purposes of the present invention, the term “chip mode” refersto a longitudinal cavity mode of an ECLD that is determined by thecavity formed between the reflective facets of the laser diode chip.During tuning of the ECLD, the modes of external cavity formed betweenthe diffraction grating and one facet of the laser diode primarilycontrol the mode of the ECLD. Occasionally, the cavity formed by thelaser diode chip can force the mode of the ECLD to change into alignmentwith the modes of the chip, creating mode hops that reduce contrastratio and are detrimental to producing strong holograms. Sometimes thestate of the laser when it is mode hopping due to the transition into acavity mode supported by the laser diode chip is referred to as a “chipmode” of the ECLD.

For the purposes of the present invention, the term “current dither”refers to quickly changing the ECLD current back and forth duringoperation of the AMC process.

For the purposes of the present invention, the term “current dithercycle” refers to one such back and forth current dithering.

For the purposes of the present invention, the term “current sweep”refers to adjusting the current for a laser over a range of currents andobserving the contrast ratios for each current value.

For the purposes of the present invention, the term “data beam” refersto a recording beam containing a data signal. As used herein, the term“data modulated beam” refers to a data beam that has been modulated by amodulator such as a spatial light modulator (SLM).

For the purposes of the present invention, the term “data modulator”refers to any device that is capable of optically representing data inone or two-dimensions from a signal beam.

For the purposes of the present invention, the term “data page” or“page” refers to the conventional meaning of data page as used withrespect to holography. For example, a data page may be a page of data,one or more pictures, etc., to be recorded or recorded in a holographicstorage medium.

For the purposes of the present invention, the terms “detector” and“sensor” refer interchangeably to any type of device capable ofdetecting or sensing something, for example, light. Exemplary detectorsor sensors include devices capable of detecting the presence orintensity of light, or a fringe pattern. Examples of detectors orsensors may include a complementary metal-oxide-semiconductor (CMOS)camera, a charged coupled detector (CCD), and/or a quad cell photodiode,etc.

For the purposes of the present invention, the terms “external cavitylaser,” “external cavity diode laser,” and “external cavity laser diode(ECLD),” (hereinafter collectively referred to as “ECLD”) refers to adevice comprising a laser diode, a diffraction grating, and at least onereflective optical element which may be used to introduce opticalfeedback into the gain medium (e.g., laser diode chip). The combinationof one or more reflective elements, possibly including the diffractiongrating, may be referred to interchangeably as a “laser cavity,”“(external) optical cavity,” “optical resonator,” or “laser resonator”(hereafter referred to collectively as “laser cavity”). This lasercavity may be used to convert a single wavelength of light emitted fromthe laser diode having a predetermined bandwidth to a specificwavelength. ECLDs may comprise a laser diode chip having one endprovided with an anti-reflection (AR) coating, while the other end hasat least a partial reflection (PR), and often a high reflection (HR),coating, with the laser cavity extending from the HR coating end to adiffraction grating (known as a “Littrow-configuration”) to provide asingle-mode of light. The wavelength of a Littrow ECLD may be tuned byrotating the grating such that it selects a different wavelength oflight within the gain of the laser diode chip. A collimating lens mayalso be provided between the AR coating and the diffraction grating, aswell as an output coupler mirror (positioned to receive the output beamfrom the diffraction grating, especially for reflective gratings). In analternative ECLD design, known as the Littman-Metcalf configuration, theexternal cavity may that comprise the HR coated end of the laser diodeand an external mirror, with the diffraction grating placed between themand used in reflection. The output of the laser may be produced by adirect reflection from the grating, while a diffracted beam from thegrating is directed to the external mirror. The external mirror providesthe feedback to the laser diode, forming the cavity. The wavelength of aLittman-Metcalf configuration ECLD may be tuned by rotating the externalmirror to selectively couple light of different wavelengths back to thelaser diode. Alternatively, the ECLD may use a laser cavity based on anoptical fiber with the optical feedback coming from a fiber Bragggrating. See also, for example, commonly-assigned U.S. Pat. No.7,495,838 (Krneta et al.), issued Feb. 24, 2009, the entire disclosureand contents of which is hereby incorporated by reference, for anillustrative ECLD having an AR coating on one facet of the diode crystaland a HR coating on the other, opposite facet of the diode crystal.

For the purposes of the present invention, the term “external cavitylaser (ECL or ECLD) data” refers to data received, or obtained, etc.,from the ECLD which may be used to determine the degree to which theECLD is (or is not) operating in a single-modes state. Such data mayinclude contrast ratio, fringe visibility, output power, wavelength,optical spectrum etc.

For the purposes of the present invention, the term “external cavitymultimode” refers to a state of an external cavity laser consisting ofmultiple longitudinal or transverse modes of the external optical cavitylasing simultaneously with non-zero optical power. One mode may bepredominant, but other modes, sometimes called side-modes, of loweroptical power may be present in the cavity and appear in the opticalspectrum.

For the purposes of the present invention, the term “external cavitysingle-mode” refers to a state of an external cavity laser consisting ofonly one longitudinal or transverse mode of the cavity. Only one modepossesses optical power, which appears as a single feature or line in anoptical spectrum

For the purposes of the present invention, the term “good hologram”refers to a hologram whose signal-to-noise ratio is within about 1 dB,and, in one embodiment of the present invention, preferably not greaterthan 0.2 dB less than the signal-to-noise ratio determined by theholographic drive parameters.

For the purposes of the present invention, the term “histogram” refersto an assembly, and/or compilation, etc., of contrast ratios measuredversus the ECLD current for many different diffraction grating angles.

For the purposes of the present invention, the terms “holographicgrating,” “holograph” or “hologram” (collectively and interchangeablyreferred to hereafter as “hologram”) are used in the conventional senseof referring to an interference pattern formed when a signal beam and areference beam interfere with each other. In cases where digital data isrecorded page-wise, the signal beam may be encoded with a datamodulator, e.g., a spatial light modulator, etc.

For the purposes of the present invention, the term “holographicrecording” refers to the act of recording a hologram in a holographicstorage medium. The holographic recording may provide bit-wise storage(i.e., recording of one bit of data), may provide storage of a1-dimensional linear array of data (i.e., a 1×N array, where N is thenumber linear data bits), or may provide 2-dimensional storage of a pageof data.

For the purposes of the present invention, the term “holographic storagemedium” refers to a component, and/or material, etc., that is capable ofrecording and storing, in three dimensions (i.e., the X, Y and Zdimensions), one or more holograms (e.g., bit-wise, linear array-wise orpage-wise) as one or more patterns of varying refractive index imprintedinto the medium. Examples of holographic media useful herein include,but are not limited to, those described in: U.S. Pat. No. 6,103,454(Dhar et al.), issued Aug. 15, 2000; U.S. Pat. No. 6,482,551 (Dhar etal.), issued Nov. 19, 2002; U.S. Pat. No. 6,650,447 (Curtis et al.),issued Nov. 18, 2003, U.S. Pat. No. 6,743,552 (Setthachayanon et al.),issued Jun. 1, 2004; U.S. Pat. No. 6,765,061 (Dhar et al.), Jul. 20,2004; U.S. Pat. No. 6,780,546 (Trentler et al.), issued Aug. 24, 2004;U.S. Patent Application No. 2003-0206320, published Nov. 6, 2003, (Coleet al), and U.S. Patent Application No. 2004-0027625, published Feb. 12,2004, the entire disclosure and contents of which are herebyincorporated by reference.

For the purposes of the present invention, the term “initial operatingpoint” refers to the combination of laser diode current and ECLDwavelength that meet the power and wavelength requested at the start ofthe AMC algorithm (within the limits of the power and wavelength range)and that satisfy the search the largest single-mode contrast ratioplateau (if this part of the AMC algorithm is enabled).

For the purposes of the present invention, the terms “laser coherencelength” and “coherence length of the laser” refer to a measure of thebandwidth of the optical spectrum of a laser or laser diode. Thecoherence length is related to the tolerable path length differencebetween the reference and data beams by the fact that a larger opticalbandwidth has a larger spectral width, and equivalently a shortercoherence length. A shorter coherence length results in a shortertolerable optical path length difference between the reference and databeams, which may manifest itself as a weaker interference pattern, andhence a weaker hologram strength, until the hologram strength reaches orapproaches zero (no hologram) when the path difference is equal to thecoherence length.

For the purposes of the present invention, the term “laser diode” refersto a laser where the active medium is a semiconductor similar to thatfound in a LED which may operate to generate, produce, etc., a laserlight (beam), and which may have a single wavelength (single-mode) ormultiple wavelengths (multimodes).

For the purposes of the present invention, the term “light emittingdiode” (LED) refers to a semiconductor diode which may be a source oflight and which may have a single wavelength or multiple wavelengths. AnLED may be used as positional light source.

For the purposes of the present invention, the terms “mode” and“longitudinal mode” refer interchangeably to a wavelength (orwavelengths) of light generated by a laser light source.

For the purposes of the present invention, the term “multimodethreshold” refers to the level of contrast ratio below which a laser istoo incoherent to produce a good hologram.

For the purposes of the present invention, the terms “multimode” and“multiple longitudinal mode” refer interchangeably to multiplewavelengths of light generated by the laser light source. For example, amulti-mode laser diode produces multiple wavelengths of light withsignificant power. FIG. 5 of commonly-assigned U.S. Pat. No. 7,397,571(Krneta et al.), issued Jul. 8, 2008, the entire disclosure and contentsof which is hereby incorporated by reference, illustrates an exemplaryfringe pattern for a multi-mode laser.

For the purposes of the present invention, the term “operating current”refers to the laser diode current at which a laser is presentlyoperating.

For the purposes of the present invention, the term “positional lightsource” refers to a source of light which may be used for determining,directly or indirectly, the position of a diffraction grating.

For the purposes of the present invention, the term “processor” refersto a device capable of, for example, executing instructions,implementing logic, calculating and storing values, etc. Exemplaryprocessors may include application specific integrated circuits (ASIC),central processing units, microprocessors, such as, for example,microprocessors commercially available from Intel and AMD, etc.

For the purposes of the present invention, the term “reading data”refers to retrieving, recovering, or reconstructing holographic datastored in a holographic storage medium.

For the purposes of the present invention, the term “recording data”refers to storing or writing holographic data in a holographic storagemedium.

For the purposes of the present invention, the term “recording light”refers to a light source used to record information, data, etc., into aholographic storage medium.

For the purposes of the present invention, the term “set pointthreshold” refers to a value of contrast ratio for a laser below whichthe AMC process of the present invention starts to adjust the current orwavelength of the laser. The set-point threshold value is higher thanthe multimode threshold value. The set-point threshold value may bedefined as a fixed offset above the multimode threshold, to ensure thatthe laser has some margin in the contrast ratio during which to adjustthe current or wavelength before declaring the laser is multimode.Alternatively, the set-point threshold value may be defined relative tothe distribution of contrast ratio values measured for laser diodecurrent and ECLD wavelength in the operating range of the current andwavelength. The set-point threshold value of the contrast ratio may bedefined such that the laser has contrast ratio values below theset-point for only a certain percentage of the possible values ofcurrent and wavelength. The percentage may be about 30%, but could alsobe much lower such as 5%.

For the purposes of the present invention, the term “single-modeoperation” refers to a laser operating above the multimode threshold.

For the purposes of the present invention, the term “single-modeplateau” refers to any region of a contrast ratio which is above (higherthan) the multimode threshold and whose slope is below a definedcriteria, such as the contrast ratio does not change more than 3 unitswith each 50 microamp change in the laser diode current.

For the purposes of the present invention, the terms “single-mode” and“single longitudinal mode” refer interchangeably to a single wavelengthof light generated by a laser light source. For example, a single-modelaser diode produces a single dominant wavelength. FIG. 4 ofcommonly-assigned U.S. Pat. No. 7,397,571 (Krneta et al.), issued Jul.8, 2008, the entire disclosure and contents of which is herebyincorporated by reference, illustrates an exemplary fringe pattern for asingle-mode laser.

For the purposes of the present invention, the term “spatial lightmodulator” refers to a data modulator device that is an electronicallycontrolled, active optical element.

For the purposes of the present invention, the term “tunable parameter”refers to a parameter which may be used to tune a laser. Tunableparameters may include the laser current, the wavelength of the outputlight, the temperature of the laser diode or collimator, the temperatureof the baseplate, the alignment of the cavity by a mechanical adjustmentmechanism, etc.

For the purposes of the present invention, the term “tunabletransmission grating” refers to a transmissive diffraction grating inwhich the particular wavelength of light reflected may be adjusted.

For the purposes of the present invention, the term “wavelength step”refers to a fixed amount that the wavelength of the output light of alaser may be adjusted in a AMC process according to one embodiment ofthe present invention.

For the purposes of the present invention, the term “wobble threshold”or “dither threshold” refers to a maximum range of values that thecontrast ratio is allowed to change with a step change in the laserdiode current (such as 50 microamps per step) before the laser makes achange in the current. For instance, in one application the wobblethreshold might be set to 3 contrast ratio units. During a currentdither cycle, the current may be stepped up and down by 50 microampsteps. If as a result of either an up or down step the contrast ratiochanges by more than 3 units, then the average current level would bestepped either up or down (depending on the direction that caused thechange in contrast ratio).

DESCRIPTION

In holographic data storage, lasers with a coherence length longer thanthe path length difference between the reference path and data path ofthe interferometer need to be used. If the optical path lengthdifference is longer than the coherence length of the laser, then thephase of light in the reference path become uncorrelated with the phaseof light in the data path, which reduces the intensity of theinterference fringes that form the hologram. In general, these lasershaving longer coherence lengths are more expensive, cumbersome, andfragile. The least expensive, most robust lasers available, laserdiodes, may not have the necessary coherence length to record (write)holograms. External cavity laser diodes (ECLDs) may be tuned so as tohave a long enough coherence length, or equivalently a more purefrequency output (referred to hereafter as a “single-mode operation”).But ECLDs may be susceptible to environmental changes such astemperature swings and air currents between the laser diode and theexternal diffraction grating, i.e., in the external laser cavity. Inorder to continuously record (write) holograms for extended periods oftime, it may be necessary to detect fluctuations in the ECLD properties(wavelength, power, contrast rato, etc.) as the environment changes andto adapt input and/or output parameters to the ECLD to keep the ECLD insingle-mode operation.

ECLDs which are correctly designed and operating properly have ahistogram of the form such as that of histogram 100 shown in FIG. 1.Histogram 100 shows the number of occurrences of each contrast ratio foran ECLD adjusted over a range of laser diode currents and a range ofECLD wavelengths. Histogram 100 has several distinct features: amultimode region indicated by double-headed arrow 112, a multimodethreshold indicated by line 114 and a single-mode region indicated bydouble-headed arrow 116. One feature is the large weighting of contrastratios in single-mode region 116. The more occurrences that an ECLD hasin region 116 of histogram 100, the better the ECLD tends to operate. Inother words, given random fluctuation of the parameters described above,the ECLD may be most likely to operate with a contrast ratio in thesingle-mode regions 116 of histogram 100. Each ECLD may have severaldifferent value regions of contrast ratios which correspond to severaldifferent behaviors of the ECLD during operation, known as the chipmode, external cavity multimode, and external cavity single-mode. Thecontrast ratio may be higher for each of these modes, respectively.Moreover, the laser may behave with a mixture of laser cavity modes ofdifferent amplitudes in optical power due to multiple external cavitymodes and multiple modes of the cavity formed by the laser diode chip.Misaligned lasers generally have histograms with two additional peaks(not shown) located to the left of peak 122 in FIG. 1 that represent thechip mode and external cavity multimode operation. Because a single-modeoperation is desirable, multimode threshold 114 is used to determine thethreshold of “good” (acceptable) versus “bad” (unacceptable) ECLDoperation. Holograms recorded (written) with an ECLD which has acontrast ratio higher than multimode threshold 108 are considered “good”(acceptable) holograms. While the contrast of the hologram interferencepattern may not suddenly disappear when the ECLD contrast ratio crossesto the left of multimode threshold 114, it may decrease rapidly fromthis point downward along the slope of histogram 100 to the left of peak122.

Typically a mode histogram of the type shown in FIG. 1 consists of arange of current covering 3 to 8 mA and a range of wavelength covering 2to 6 nm.

Generally, if the histogram resembles that shown in FIG. 1 then thelarge feature at the right side of the histogram indicates an ECLD thatpredominantly operates in a single-mode of operation. The width of thefeature indicates the range of behavior the laser exhibits. For example,a narrow feature indicates a laser with consistently single-modebehavior. The maximum value of the contrast ratio indicates howwell-aligned the mode sensor is, as well as details of the suppressionof incoherent light during single-mode operation.

Embodiments of the present invention may use a certain process, known asan Automatic Mode Control, or “AMC,” process to tune ECLDs used inholographic data storage in order to achieve longer periods of stablesingle-mode operation. The AMC process is thus used to keep the ECLDtuned to operate as a single-mode laser. The tunable parameters mayinclude the laser current and the output light wavelength. Adjusting thelaser diode current alters the temperature of the ECLD by a smallamount, on the order of about to 3 to 4° C. per milliamp of current. Theoutput light wavelength may be adjusted by changing the diffractiongrating angle for the laser using techniques such as those described incommonly assigned U.S. patent application Ser. No. 12/457,498, entitled“SYSTEM AND DEVICES FOR IMPROVING EXTERNAL CAVITY DIODE LASERS USINGWAVELENGTH AND MODE SENSORS AND COMPACT OPTICAL PATHS” (Ensher et al.),filed Jun. 12, 2009, the entire contents and disclosure of which ishereby incorporated by reference.

In other embodiments, the present invention may use laser diodetemperature, or laser baseplate temperature, as part of the AMC process.Alternatively, the alignment of the laser cavity might be part of theAMC process whereby one or more mechanical mechanisms might adjust thealignment of the laser diode or collimator relative to the diffractiongrating.

ECLDs that use the AMC process may contain a linear sensor array tomeasure the degree of the ECLD being in a single-mode state, asdescribed in commonly assigned U.S. patent application Ser. No.12/457,498, entitled “SYSTEM AND DEVICES FOR IMPROVING EXTERNAL CAVITYDIODE LASERS USING WAVELENGTH AND MODE SENSORS AND COMPACT OPTICALPATHS” (Ensher et al.), filed Jun. 12, 2009, the entire contents anddisclosure of which is hereby incorporated by reference. The metric usedto measure the degree of the ECLD being in a single-mode state may bethe contrast ratio, and/or interchangeably, the fringe visibility. Thenumber for the contrast ratio may be in the range of from 0(representing the output from a highly multimode source, such as thesun) to 1 (representing the output from a perfect, single-frequencysource). For lasers with narrow spectral linewidths and long coherencelengths, this metric may be above about 0.8 and for small cavity,non-selective gain lasers, such as an individual diode, this number maybe below about 0.2.

When tuning the current in an ECLD, the contrast ratio metric may rangefrom a relatively low number (representing a multimode operation) to arelatively high number (representing a single-mode operation). Anillustrative plot of such tuning is shown in FIG. 2, and is indicatedgenerally as 200. As shown in FIG. 2, plot 200 has several “plateaus,”indicated as 212, 214, 216, and 218, as well as several “valleys,”indicated as 222, 224, 226, and 228 where the contrast ratio isrelatively constant as the diode current changes. The structure of thecontrast ratio versus laser current plot may be different depending uponany very small variation of temperature, air currents, grating angle,vibration in the system, etc. For example, the structure may vary in thefollowing ways: The depth of the valleys in the contrast ratio vs.current may be increased. The length of the contrast ratio plateaus maybecome shorter or longer. The depth and height of the valleys andplateaus, respectively, may remain unchanged, but the number of valleysand plateaus may increase as the number of transitions from low-to-highincreases, indicating more frequent mode hops).

If the contrast ratio is measured versus the ECLD current for manydifferent angles of the diffraction grating, a histogram of differentmeasured contrast ratios may be assembled, compiled, constructed, and/orobtained, etc., such as the one illustrated in FIG. 2 and generallyindicated as 200. With a large enough number of such measurements, thehistogram of contrast ratios for any ECLD may remain the same,regardless of variations in current, wavelength, temperature, and/or aircurrents, etc. In one embodiment of the present invention, a sufficientnumber of measurements is about 500 to 5000 This assembled compiled,constructed, and/or obtained, etc., histogram 200 may therefore be onemethod for determining how well the ECLD tends to work what range thecontrast ratio metric may span on a single ECLD; and/or what contrastratio threshold may be required for holography, etc.

In one embodiment, the AMC process of the present invention comprisestwo distinct parts. The first part of the AMC process is related tochoosing the best initial operating point in both wavelength and currentprovided for the laser. The second part of the process is related totracking the best operating point if there are variations intemperature, air current, and/or any other unknown mechanism whichcauses the laser to shift the preferred operating point of the laser.

In one embodiment, the AMC process of the present invention is designedto keep a laser in single-mode operation, without firmware assistance,by bumping the current and wavelength values small amounts if thecontrast ratio falls below a control set-point. The hardware alsocontains support for searching a range of currents to find the center ofa range in which the laser is in single-mode, which is an optimizationthat may be enabled or disabled depending on what else is occurring withthe laser and how much time is available.

In one embodiment, when the AMC process is initialized, the user orholographic data storage drive knows the desired operating power andwavelength. The first two steps of the initialization portion of the AMCprocess are to swing (pivot or rotate) the diffraction grating to theproper angular location to lase (operate) at the desired initialwavelength, and then to either increase or decrease (adjust) the ECLDcurrent until the desired initial power output is achieved. Thewavelength and power of ECLD are both measured with external sensors asdescribed and shown in commonly assigned U.S. patent application Ser.No. 12/457,498, entitled “SYSTEM AND DEVICES FOR IMPROVING EXTERNALCAVITY DIODE LASERS USING WAVELENGTH AND MODE SENSORS AND COMPACTOPTICAL PATHS” (Ensher et al.), filed Jun. 12, 2009, the entire contentsand disclosure of which is hereby incorporated by reference. FIGS. 1, 2,5, 6, and 10 of Ensher et al. show embodiments of a wavelength sensor.FIGS. 11 and 12 of Smith et al. show embodiments of a mode sensor. FIG.13 of Ensher et al. shows how a beam transmitted through a mode sensorembodiment reaches a power sensing photodiode, mounted to a printedcircuit board as shown in FIG. 13. FIG. 23 of Ensher et al. showscreating and redirecting non-output beams from a grating for sensinglaser properties such as mode, wavelength and power. Even though thecurrent and diffraction grating angle of the ECLD are completely definedby the desired power and wavelength respectively, it may still benecessary to adjust these parameters to make the ECLD operate withsingle-mode behavior.

The next step of the initialization portion of the AMC process is tosweep the ECLD current by a small amount around the initial operatingcurrent provided to the ECLD while measuring the contrast ratio. Achange in ECLD current may also cause a change in the ECLD output power,but the amount of the ECLD current sweep may be chosen to be smallenough so that the hologram signal amplitude and signal-to-noise ratiomay not be affected or are minimally affected, such as by changing thesignal-to-noise ratio by less than about 1 dB. During the ECLD currentsweep operation, shown generally by plot 300 in FIG. 3, the ECLD currentis first swept up, and then swept down, generating two sweep curvessimilar to those shown in FIG. 3, which are indicated, respectively, asup current sweep 312 and down current sweep 314. Also shown in FIG. 3 isthe multimode threshold, which is indicated by solid horizontal line322, and a second, higher threshold, the set point threshold, which isindicated by dashed line 324. Any region of the contrast ratio that isabove (higher than) multimode threshold line 322 is known as asingle-mode plateau. Up current sweep 312 includes three single-modeplateaus 332, 334 and 336. Down current sweep 314 also includes threesingle-mode plateaus 342, 344 and 346. An overlapping single-modeplateau 352 is formed where single-mode plateau 332 and 342 overlap. Anoverlapping single-mode plateau 354 is formed where single-mode plateau334 and 344 overlap. An overlapping single-mode plateau 356 is formedwhere single-mode plateau 336 and 346 overlap. Overlapping single-modeplateaus 352, 354 and 356 have respective widths shown by respectivedouble-headed arrows 362, 364 and 366. The AMC process first looks foran overlapping single-mode plateau higher than a set-point thresholdline 324 that has a power output within the acceptable power range. Inone embodiment of the present invention, the acceptable power range maybe provided by the user or the holographic drive. The range of opticalpowers accepted may be determined by the range of hologramsignal-to-noise ratios produced by the range of optical powers, where anacceptable signal-to-noise ratio (SNR) range might be 1 dB.

If one or more such overlapped single-mode plateaus exist, such asoverlapped single-mode plateaus 352, 354 and 356, the ECLD current isset to the middle of the widest single-mode overlapped plateau, (e.g.,midpoint 358 of overlapped single-mode plateau 354 in FIG. 3). Upcurrent sweep also includes valleys 372, 374 and 376. Down current sweep314 also includes valleys 382, 384 and 386.

If single-mode plateaus exist, but none of the single-mode plateausoverlap in the current range swept, the ECLD current is then set to acurrent, the operating current, which is at or proximate the midpoint ofthe widest single-mode plateau. The current is also set by approachingthe operating current in the direction of current sweep that producesthe largest single-mode plateau. For example, if the widest single-modeplateau is found during down current sweep 312, then the ECLD current isreset to the highest current value in sweep 312, and then decreased toreach the midpoint of that widest single-mode plateau. The finalpossibility is that no single-mode plateaus above (higher than) theset-point threshold and within the power tolerance are found. In thiscase, the wavelength is tuned by one or more small steps, where thetotal wavelength change is within the acceptable wavelength range, withthe current sweep procedure, described above, then being repeated. Inone embodiment, the wavelength steps may be about 10 picometers each.

When the initialization part of the AMC process is finished and anoperating current and wavelength have been chosen, the ECLD should beoperating in a single-mode regime. The job of finding a single-modeoperating point (e.g., midpoint 358) would then be complete if the ECLDwere not so sensitive to variations in temperature, air currents,potentially vibrations, etc. However, over time, the plateau structure,such as that shown by 300 in FIG. 3, of the ECLD tends to change. If thesingle-mode plateau currently being used for the single-mode operatingpoint drifts to a higher (or lower) current range, as well as far enoughaway from the original current range so that the chosen operatingcurrent falls off the single-mode plateau being used, the ECLD may ceaseto be in a single-mode operation.

The second part of the AMC process is used to track changes in thisplateau structure (i.e., the structure of the single-mode plateaus,either in size or position), and adjusting the ECLD current based on thetracked changes to follow a single-mode operating point for the ECLD.One embodiment of a mechanism used to track changes in the single-modeplateau structure is a current dither. During the operation of the AMCprocess, the ECLD current may be quickly dithered back and forth (a“current dither cycle”) one or more times between two very similarcurrent values, i.e. current values separated by a current step, whilemeasuring the contrast ratio. For example, the amount of current ditherused may be 50 microamps (mA), with the contrast ratio being measured,for example, eight (8) times for each current dither cycle to provide anaverage or mean contrast ratio value for each cycle of eightmeasurements. In other embodiments, only one to three samples of thecontrast ratio may be used to increase the speed of response of thecurrent dither cycle. If the difference in two consecutive measurementsof the average or mean contrast ratio is greater than a chosen orselected dither threshold, the operating current may be changed to acurrent value providing a higher contrast ratio. The dither thresholdmay be chosen to be small, i.e. between about 3 and 7. A small ditherthreshold forces the AMC process to change the ECLD current more oftenwhich may lead to instabilities in the laser performance, such asrapidly changing contrast values as the AMC attempts to dither on ornear the edge of a contrast ratio plateau. If the wobble threshold ischosen to be large i.e. between about 10 and 30, this may keep thesingle-mode operating point more stable, but may not track plateaushifts, to a higher or lower current range, as quickly. For a contrastratio scale of from 0 to 1, the wobble threshold may be generally set tobetween about 0.003 and about 0.01.

In effect, the embodiment of this current dither mechanism or proceduredescribed above allows the ECLD to test the contrast ratio both aboveand below the operating current. If the current dither mechanism orprocedure determines that one direction is significantly better, i.e.produces an increase in the contrast ratio greater than the ditherthreshold, then the ECLD current is changed to utilize the new andbetter operating point. If neither direction is much better then theoperating current must be on top of a flat single-mode plateau, andnothing is (needs to be) changed.

An embodiment of a current dither mechanism or procedure for trackingthe single-mode plateaus works if these single-mode plateaus arechanging at a slower rate than the measurement of two consecutivecurrent dither cycles. Even with a dedicated field-programmable gatearray (FPGA) making the measurements on a millisecond time scale, thesingle-mode plateaus may still drop away faster into adjacent valleysthan the AMC process may track. Usually, these rapid changes in thesingle-mode plateau structure of the ECLD are due to some mechanicalvibration of the diffraction grating, or the laser diode and collimatorlens. Rapid changes in the single-mode plateau structure can also occurimmediately after a sudden change in temperature of the laser, or due totransient changes in the mechanical alignment of the grating laser diodeor collimator that might occur due to shock, or an accumulation ofvibration or temperature that induces a sudden relaxation of mechanicalstress.

If the AMC process automatically uses a current dither mechanism orprocedure to push the process to the largest single-mode plateaus, theAMC process can get lost in the valleys (e.g., 372, 374, 376, 382, 384and 386), which are also flat enough to keep the ECLD current fromchanging. To avoid this pitfall, the AMC process ignores the ditherthreshold if the contrast ratio drops below the set-point threshold 324.In this scenario, the current dither is used to determine the directionto change the current regardless of the change in the contrast ratiorelative to the dither threshold (see FIG. 3). If no single-mode plateauis found within a certain amount of time, the AMC process may bere-initialized to find the nearest single-mode operating current andwavelength. The re-initialization process may consist of returning thelaser to the initial power and wavelength requested at the start of AMC.With well-aligned ECLDs, such as the one used to make histogram 100shown in FIG. 1, any random walk of the ECLD current quickly locates asuitable single-mode plateau.

FIG. 4 illustrates one embodiment of an AMC process 402 of the presentinvention. When process 402 is off or not being used, process 402 is inan idle state 412. At step 414, a laser is set to an initial wavelength.At step 418, process 402 attempts to set an initial power output bysetting the initial power of the laser. If the desired initial poweroutput of the laser cannot be achieved because mode hops cause the powerto fluctuate larger than the maximum power range, the laser wavelengthis bumped at step 422 by a wavelength step to a new current wavelength.Once the laser wavelength is bumped at step 422, step 418 is thenrepeated as indicated by arrow 424. Once the initial power output is setto the starting requested power for the laser, automatic mode controlprocess 402 conducts a current sweep to find an SM plateau at step 426.If no SM plateaus are found, the current wavelength is bumped at step430 by a wavelength step to a new current wavelength. Once the laserwavelength is bumped, step 426 is then repeated as indicated by arrow432. Once one or more SM plateaus are found, the current is set to avalue in the middle of the largest SM plateau and step 436 is conducted.At step 436, the wavelength is measured. If the measured wavelength isoutside the AMC wavelength range, then step 414 is performed asindicated by arrow 440. Then the power output of the laser is measuredat step 436. If the measured output power is outside the allowed powerrange, then step 418 is performed as indicated by arrow 442. Once themeasured wavelength is inside the AMC wavelength range and the measuredoutput power is inside the allowable power range, the contrast ratio ischecked at step 446. If at step 446 the contrast ratio is above theset-point threshold, the contrast ratio is simply checked again asindicated by arrow 458. If at step 446, the contrast ratio is below thecontrol set point threshold, then, the current is adjusted at step 452and the operating conditions i.e. the measured AMC wavelength andmeasured output power, are checked again at step 436 as indicated byarrow 454. If at step 446, the current is beyond the maximum currentrange, the wavelength is bumped at step 430 as indicated by arrow 456,and step 426 is performed as indicated by arrow 432.

Although the process of FIG. 4 is described above employing a singlecurrent sweep at step 426, in one embodiment, the AMC process of thepresent invention conducts two or more current sweeps at step 426 tofind one or more overlapped SM plateaus. Once one or more overlapped SMplateaus are found, the current is set to a value in the middle of thelargest overlapped SM plateau and step 436 is conducted.

In one embodiment of the present invention, the user chooses or theholographic storage drive will know, based on information stored inmemory, the optical power that is requires for writing good holograms.Based on this optical power, the user or holographic storage drive canissue the command to enter the AMC process, because the laser knows howto implement a power request into a current request. Alternatively, thecalibration for power to current may reside in the drive or be chosen bythe user, so that the current command is sent to the laser.

In one embodiment of the present invention, the initial wavelength isset using a value that is stored in the firmware for a processorimplementing the AMC process of the present invention.

In one embodiment of the present invention, the starting requested powerusing a value that is stored in the firmware for a processorimplementing the AMC process of the present invention. If the setinitial power procedure fails because of an iteration error (i.e. thepower was not able to be set in the programmed number of iterations),the wavelength may be bumped by a wavelength step before trying to setthe power again.

In one embodiment of the present invention, if the search current rangeoptimization is enabled, a processor implementing the AMC processselects an optimized current value within a maximum current range.Otherwise, the processor stays with the current found during the setinitial power step. If the search current range optimization is enabledand a valid operating range is not found, the wavelength may be bumpedby a wavelength step before the processor searching the current rangeagain.

In one embodiment of the present invention, the check operatingconditions procedure step performs several functions. If the measuredwavelength is outside of the expected wavelength range, the processorimplementing the AMC process goes back and sets the wavelength to bewithin the defined precision. Note that the “expected wavelength range”is a range around the current expected wavelength, which may not be thesame as the initial wavelength because of wavelength bumps. Thewavelength check has the highest priority. If the measured power isoutside of the expected power range (as defined by a maximum power rangememory bit), the processor goes back and set the power to be within thedefined precision. The power check has the second highest priority. Ifboth wavelength and power are within the maximum wavelength and powerrange, respectively, the processor continually checks the contrastratio. If contrast ratio drops below the control set-point, the currentis adjusted. Note that the wavelength and power are checked during eachmode detection, and the values are updated if they are found to beoutside the valid range at any time. There are several real-time controlbits that influence the above steps. The firmware for the processor hasthe ability to enable or disable the current and wavelength updates tocorrespond with writing/reading holograms. Since current and wavelengthupdates during writes/reads may cause errors, the updates need to bedisabled during the write/reads. If firmware has disabled the currentupdate operation, the processor simply goes back to checking the modeeven if it determines that the contrast ratio is below the controlset-point but above the multimode threshold. If the current updateoperation is enabled but the wavelength update operation is disabled,the processor returns to checking the mode even if it wants to updatethe wavelength. Only when firmware determines that updates are safe andthe operations are enabled does the processor change the current and/orwavelength.

In one embodiment of the present invention, updates of wavelength andpower may be controlled in real-time via input pins to a processor thatimplements the AMC process of the present invention. When the processordetermines that the laser is in a single-mode state, based on data fromthe sensors for the laser, a current update memory bit for the processoris set to “0”, so that current updates are not allowed. The AMC wobbleprocess does not toggle back and forth between currents, but insteadjust monitors the contrast ratio. When the processor determines that thelaser is no longer in a single-mode state, based on data from thesensors for the laser, the current update memory bit for the processoris set to “1” and the wobble process runs and the current is able to beupdated. Note that the wavelength is not updated if current updatememory bit is set to “0”. Wavelength updates may be restricted inreal-time via a wavelength update memory bit. If the wavelength updatememory bit is set to “0”, the wavelength is not be bumped by theprocess. (Note that, if the process wants to bump the wavelength and isunable to, an AMC failure may result.) If this wavelength update memorybit is set to “1”, the wavelength is able to be bumped by the processor.Wavelength bumping is also enabled or disabled via a tune wavelength bitfor the processor. If the tune wavelength bit is a “0”, the updatewavelength memory bit is ignored. If wavelength tuning bit is a “1” andwavelength update bit is also a “1”, then a wavelength bump may takeplace. If a current increment or decrement would push the actual currentbeyond the valid range specified by the current range parameter, theprocessor has the option of incrementing the wavelength by a wavelengthstep, and then reverting to current control to keep the laser insingle-mode at the new wavelength. This only continues until anincrement of the wavelength pushes the wavelength of the output lightoutside the maximum wavelength range. Once outside this range, theprocessor stops implementing the AMC process and the processor sends andinterrupt signal to firmware.

In one embodiment of the present invention, in the “search current rangefor SM plateau” step, for each wavelength, the range of currents withthe maximum current range may be searched to find the largest rangewhere the laser is in single-mode and the measured power is within avalid range. The processor then sets the current to the middle value inthis range with the hope that the laser stays in single-mode operationfor a long time. When this optimization is enabled via an optimizecurrent bit, the processor begins with the lowest current value in therange. The processor then increments the current by a current stepthroughout the range defined by the maximum current range, checking thelaser mode and power after each current update. The processor keepstrack of and stores the starting and ending current values for thelargest range in which the laser is in single-mode and the measuredpower for the laser is within the valid range specified by the maximumpower range. The processor then decrements the current from the maximumcurrent value in the maximum current range back to the starting current,also keeping track of and storing the starting and ending current valuesfor the largest range in which the laser is in single-mode and themeasured power of the laser is within the valid range specified bymaximum power range. Once the sweep has been completed in bothdirections, the two ranges of single-mode operation are compared and thelarger of the two is chosen. The processor then determines the centervalue of the selected range. The final step in this procedure is for theprocessor to set the current to the center value of the optimized range.This is accomplished by stepping the current in the direction of currentramp that produced the larger plateau until the center current value isreached.

As mentioned above, in one embodiment of the present invention, thewavelength and power are continuously measured during each contrastratio calculation. If the wavelength or power falls out of a validrange, a processor implementing the AMC process of the present inventionmay go back to an earlier state to set the wavelength or power backwithin a valid operating range. While the wavelength and power ismeasured concurrently with each contrast ratio calculation, the choiceof setting the wavelength or power back into a valid range is controlledby firmware. If a check wavelength enable bit is “1”, then the processorsets the wavelength back to the expected value for the wavelength if themeasured wavelength is outside the maximum range specified by a maximumwavelength range memory bit. Note that the maximum wavelength rangememory bit specifies a range around the last wavelength set. Thatwavelength may be the initial wavelength set at the start of theprocessor implementing the AMC process or the wavelength may be theinitial wavelength incremented by one or more wavelength steps. Notethat the maximum wavelength range value should be more than twice aslarge as the laser wavelength precision value for the laser, or else theAMC process may enter an infinite loop trying to continuously reset thewavelength. If a check power enable memory bit is “1”, then theprocessor sets the power back to its expected value if the measuredpower is outside the maximum specified by the maximum power range memorybit. Note that the maximum power range memory bit specifies a rangearound the initial power. Also note that the maximum power range valueshould be more than twice as large as the laser power precision value,or else the AMC process could enter into an infinite loop trying tocontinuously reset the power.

In one embodiment of the present invention, the search current rangeoptimization step described previously sets the current to the middle ofthe largest single-mode plateau. But plateaus drift and sometimes erodeaway with temperature and other operating condition changes. The mainAMC loop (steps 430, 436, 446 and 452 of flowchart 402) attempts to keepthe laser operating point away from the edge of a single-mode plateau,allowing the laser to respond to changes and keeping it in single-modeoperation longer, reducing the number of reported multimode errors. Themain AMC loop has two (2) parts. The first part is a plateau trackerprocess that attempts to keep the current set to the middle of theplateau as it drifts or erodes. The two (2) currents at the edges of theplateau are saved. At the beginning of each write of a book ofholographic pages of data, the current is changed to the two (2) edgesand contrast ratio is measured again at both edges. If the contrastratios at both edges are still good, i.e. above the set-point threshold,no change is made. If one contrast ratio is good and the other bad, itis assumed the plateau is drifting and the two (2) edge currents areboth bumped one (1) step in the direction of the good contrast ratio bya current step. If both are bad, it is assumed the plateau is erodingand the edge currents are both bumped one (1) step towards the middle ofthe plateau. The process preferably loops eight (8) times to check andadjust the plateau edges. The current is then set to the middle betweenthe new plateau edges and the contrast ratio is checked there. If good,the current is left at the plateau mid-point. If bad, the current isrestored to what it was before the plateau tracker process started. Thesecond part is a process that adjusts the current during book writes ifcontrast ratio drops below the control set-point. It works bycontinually checking the contrast ratio measured at the present current.The checking process may examine more than one sample of the contrastratio and in certain embodiments may check the average contrast ratio ofthe most similar 2 out of every 3 measurements of the contrast ratio inorder to suppress transient or erroneous measurements. If the average oftwo contrast ratio values is below the control set-point, current isbumped one (1) step, in whichever direction improved contrast ratio inthe past. The contrast ratio is measured at the second current and thetwo (2) values are compared. The laser current is then adjusted in thedirection of the larger contrast ratio. If contrast ratios are below thecontrol set-point but above the multimode set-point, current is adjustedby one (1) step. If either is below the multimode set-point, current isadjusted two (2) steps.

The processor implementing the AMC process of the present invention maybe any type of suitable processor. In one embodiment, the processor is afield-programmable gate array (FPGA).

In the AMC process shown in FIG. 4 and described above, the values forthe initial wavelength, starting requested power, wavelength step,wavelength range, initial power output, allowable power range, and othervalues used in the AMC process may be stored on a memory chip or othertype memory device in a holographic storage device. In anotherembodiment of the present invention, these values may be set by a user.In one embodiment of the present invention, these values may be storedon a memory chip or other type of memory device in a laser module thatmay be mounted in a holographic storage device. In one embodiment of thepresent invention, these values may be stored on a memory chip or othertype of memory device in storage module of holographic storage device inwhich laser may be mounted.

In one embodiment of the present invention, instructions forimplementing the AMC process of the present invention may be stored in amemory chip of a holographic storage device that includes a processorfor implementing the AMC process. An example of a holographic storagedevice including a memory chip that may be programmed with theinstructions for implementing the AMC process are described and shown inthe Tapestry™ 300r storage drive from InPhase Technologies. In oneembodiment of the present invention instructions for implementing theAMC process of the present invention may be stored in memory chip of alaser module a holographic storage device that includes a processor forimplementing the AMC process. Examples of ECLD modules including memorychips that may be programmed with the instructions for implementing theAMC process are described and shown in the NUV601E from NichiaCorporation using an M25P40 programmable read only memory chip from STMicroelectronics, as well as FIG. 3 of U.S. Pat. No. 7,397,571 (Krnetaet al.), issued Jul. 8, 2008, and FIG. 1 of U.S. Patent Application No.2007/0223554 (Hunter et al.), filed Mar. 9, 2007, the entire disclosureand contents of which are hereby incorporated by reference. In oneembodiment of the present invention instructions for implementing theAMC process of the present invention may be stored in memory chip of astorage module a holographic storage device in which a laser module maybe mounted.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

Although the present invention has been fully described in conjunctionwith several embodiments thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. An apparatus comprising: a laser having a laser current and an outputlight having a wavelength; and a processor for determining if the laseris operating in a single-mode state and for determining the degree towhich one of one or more tunable parameters for the laser must beadjusted so that laser operates in a single-mode state if not operatingin a single-mode state, wherein the one or more tunable parametersinclude the following parameters: the laser current and the wavelengthof the output light.
 2. The apparatus of claim 1, further comprising acurrent control system for adjusting the laser current.
 3. The apparatusof claim 2, further comprising a wavelength control system for adjustingthe wavelength of the output light.
 4. The apparatus of claim 1, furthercomprising a wavelength control system for adjusting the wavelength ofthe output light.
 5. The apparatus of claim 1, further comprising amemory device for storing information for a multimode threshold and/or aset-point threshold for the laser, wherein the information is used todetermine if the laser is operating in a single-mode state.
 6. Theapparatus of claim 1, wherein the laser is a diode laser.
 7. Theapparatus of claim 1, wherein the laser is an external cavity laser. 8.The apparatus of claim 7, wherein the laser is an external cavity laserdiode.
 9. The apparatus of claim 1, further comprising one or moresensors for sensing at least a portion of the output light to providedata to the processor on the degree to which the external cavity laseris operating in a single-mode state.
 10. The apparatus of claim 1,further comprising a holographic storage medium for storing hologramsrecorded by the laser.
 11. A method comprising the following steps: (a)determining if a laser is operating in a single-mode state, the laserhaving a laser current and an output light having a wavelength; and (b)if the laser is determined to not be operating in a single-mode state instep (a), determining the degree to which one of one or more tunableparameters for the laser must be adjusted so that the laser operates ina single-mode state if not operating in a single-mode state, wherein theone or more tunable parameters include the following parameters: thelaser current and the wavelength of the output light.
 12. The method ofclaim 11, further comprising the following step: (c) adjusting the lasercurrent so that the laser operates in a single-mode state based on thedegree to which the laser current must adjusted determined in step (b).13. The method of claim 12, further comprising the following step: (d)determining the current range for an single-mode plateau for a singlecurrent sweep for the laser, wherein step (c) comprises setting acurrent for the laser at approximately the midpoint of the current rangefor the single-mode plateau.
 14. The method of claim 13, wherein thesingle-mode plateau is the widest single-mode plateau for the singlecurrent sweep.
 15. The method of claim 12, further comprising thefollowing step: (d) determining the current range for an overlappingsingle-mode plateau for two or more current sweeps for the laser,wherein step (c) comprises setting a current for the laser atapproximately the midpoint of the current range for the single-modeplateau.
 16. The method of claim 15, wherein the overlapping single-modeplateau is the widest overlapping single-mode plateau for the two ormore current sweeps.
 17. The method of claim 15, wherein at least one ofthe current sweeps is an up sweep and at least one of the current sweepsis a down sweep.
 18. The method of claim 11, further comprising thefollowing step: (c) adjusting the wavelength of the output light so thatthe laser operates in a single-mode state based on the degree to whichthe wavelength of the output light must adjusted determined in step (b).19. The method of claim 18, wherein step (c) comprises bumping thewavelength of the output light by a wavelength step if the output powerfor the laser cannot be set to selected power due to mode hops causingthe output power to fluctuate beyond a specified range.
 20. The methodof claim 11, further comprising the following steps: (c) adjusting thecurrent and the wavelength of the output light so that the laseroperates in a single-mode state based on the degree to which the lasercurrent must be adjusted and the degree to which the wavelength of theoutput light must adjusted determined in step (b).
 21. The method ofclaim 11, wherein the laser is a diode laser.
 22. The method of claim11, wherein the laser is an external cavity laser.
 23. The method ofclaim 22, wherein the laser is an external cavity laser diode.
 24. Anapparatus comprising: a holographic storage medium for recordingholograms using a tunable laser having a laser current and an outputlight having a wavelength; and a processor for determining if the laseris operating in a single-mode state and for determining the degree towhich one of one or more tunable parameters for the laser must beadjusted so that laser operates in a single-mode state if not operatingin a single-mode state, wherein the one or more tunable parametersinclude the following parameters: the laser current and the wavelengthof the output light.